STUMPERS CCLXIV

Adding a letter to a one syllable word can create a two or three-syllable word -as in crate to create or rode to rodeo. But can you think of a four letter, two-syllable word that changes to a five-letter, one-syllable word by the addition of a single letter?

HINTS:
1. The extra letter is added at the front of the word.
2. The four-letter word names a type of illness.

Answer:

AGUE - VAGUE